What Is Chronic Lymphocytic Leukemia (CLL)?

Oncologist With Leukemia Patient
Chronic Lymphocytic Leukemia. Leif Skoogfors / Getty Images

Chronic lymphocytic leukemia (CLL) is the most common type of leukemia in adults in the Western countries. It occurs when there is damage to the genetic material (mutation) to a cell that would normally develop into a type of white blood cell called a lymphocyte​.

CLL is different than other types of leukemia because the genetic mutation not only causes an uncontrolled growth of lymphocytes in the marrow, but it also results in cells that do not follow the normal pattern of natural cell death.

This leads to an increased number of CLL lymphocytes in the bloodstream.

In about 95% of cases of CLL in the West, the type of lymphocyte that is affected is a B- lymphocyte (B- cell CLL). T-cell CLL is more common in areas of Japan, and only accounts for about 5% of cases in the U.S.

The abnormal lymphocytes in chronic leukemia are more mature than those seen in acute leukemia. Because they are more mature, they are able to do many of the functions of normal lymphocytes. As a result, chronic leukemia can go untreated for long periods before the patient would notice any symptoms.

There are times when a healthy person may have a high lymphocyte count, and this does not mean they have leukemia. These infection-fighting cells naturally go into high production rates in times of illness. In the case of CLL, an overproduction of one kind of lymphocyte occurs (depending on where the genetic mutation originally happened) and the cells, while mature, show abnormal traits.

An important job of healthy B lymphocytes in the body is to produce immunoglobulins, which are proteins that help to fight off infection. In CLL, the abnormal lymphocytes are unable to produce immunoglobulins (or “antibodies”) that work properly, and also prevent the non-cancerous lymphocytes from producing effective antibodies.

Because of this, people with CLL often experience frequent infections.​

Signs and Symptoms of CLL

For some patients, CLL is very slow growing and the signs may be almost undetectable. Even when the numbers of lymphocytes reach high levels in the blood, they often do not impact the production of other types of blood cells such as red cells and platelets.

Patients with more aggressive types of CLL or those that have more advanced disease may show signs of burden on the bone marrow production of other cell types as well as symptoms associated with enlarged lymph nodes.

CLL patients may experience:

  • Weight loss
  • A generally “unwell” feeling
  • Night sweats
  • A tired feeling that is not relieved with a good night sleep
  • Swollen lymph nodes
  • Frequent infections
  • Pain from the pressure of enlarged lymph nodes on surrounding organs
  • Bleeding problems

These symptoms can also be the sign of other, less serious conditions. If you are worried about any changes to your health, you should seek guidance from your doctor.

Is CLL a Leukemia or a Lymphoma?

Lymphocytic leukemia and lymphoma both occur when lymphocytes multiply in an uncontrolled manner.

Both conditions can have swelling of the lymph nodes and both can show excessive lymphocytes in the blood. In fact, the difference between these two diseases is primarily in name only.

A patient is said to have CLL when they have more lymphocytes in the circulation and less swelling of the nodes. Conversely, patients who have very enlarged nodes, but near normal white cell counts in the blood are described as having small lymphocytic lymphoma (SLL), a type of non-Hodgkin lymphoma.

In many cases, the terms CLL and SLL are used interchangeably as they are essentially the same disease presented in a different way.

Risk Factors for CLL

CLL tends to be a leukemia of older individuals, with the average age at diagnosis being 65. Ninety percent of people diagnosed with CLL are over the age of 40.

Similar to other types of leukemia, scientists do not know exactly what causes CLL, but there are a few factors which appear to put some people at a higher risk of developing it:

  • Exposure to certain toxic chemicals, such as pesticides and herbicides
  • Infection with certain bacteria or viruses such as human T- lymphocytotropic virus (HTLV-1)
  • Having a first-degree relative, such as a sibling, with CLL may increase risk by three to four times

Unlike other types of leukemia, exposure to radiation does not seem to be a risk factor.

While these factors may increase a person’s likelihood of getting CLL, many people with these risk factors will never get leukemia, and many with leukemia have no risk factors.

Summing it Up

Chronic lymphocytic leukemia (CLL) occurs when damage to the DNA of a cell causes uncontrolled multiplication and an overabundance of lymphocytes. This will lead to elevated numbers of lymphocytes in the circulation, which may also collect in the lymph nodes. In most cases, people who have CLL will show few symptoms as the disease may be very slow to progress.


Chirorazzi, N., Rai, K., Ferrarini, M. “Mechanism of Disease: Chronic Lymphocytic Leukemia.” New England Journal of Medicine 2005; 352:804-15.

Lin, T., Byrd, J. “Chronic Lymphocytic Leukemia and Related Chronic Leukemias” in Chang, A., Hayes, D. Pass, H. et al. eds. (2006) Oncology: An Evidence- Based ApproachSpringer: New York. pp. 1210- 1228.

Hillman, R., Ault, K. (2002) Hematology in Clinical Practice 3rd ed. McGraw-Hill: New York.

Zent, C., Kay, N. “Chronic Lymphocytic Leukemia: Biology and Current Treatment” Current Oncology Reports 2007; 9: 345-352.

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